1. Field of the Invention
The present invention relates to a motorcycle tire for an off-road.
2. Description of the Related Art
A surface of an off-road such as a forest or a field has an undulation. For this reason, an excellent following property to a road surface is required for a motorcycle tire (hereinafter referred to as a tire) to be attached to a motorcycle which runs over an off-road. A tire (a radial tire) including a carcass having a radial structure is attached to the motorcycle. A following property to a road surface of the tire is more excellent than that of a tire (a bias tire) including a carcass having a bias structure.
The tire includes a tread constituted by a crosslinked rubber, a pair of beads and a carcass laid over both of the beads. The tread has a block pattern. A groove formed on the tread divides a tread surface into a large number of blocks so that the block pattern is constituted. An external surface of the block mainly comes in contact with a smooth road surface. A part of the tire is buried in a soft ground and the block sweeps mud. In this case, the block contributes to a tractive force.
The carcass includes a ply having a large number of cords provided in parallel. An absolute value of an angle formed by each of the cords with respect to an equator plane is 65 to 90 degrees. The cord is constituted by a polyester fiber, a nylon fiber, a rayon fiber, a polyethylene naphthalate fiber or an aramid fiber.
EP0835766 (Japanese Laid-Open Patent Publication No. 10-114203) has disclosed a tire which is excellent in a stability and a cornering performance. In the tire, an angle with respect to an equator plane and an elongation in a carcass cord included in a carcass ply are regulated properly.
The cord included in the carcass is usually formed by twisting two yarns. A modulus of a cord constituted by a yarn formed of an aramid fiber is higher than that of a cord constituted by a yarn formed of a polyester fiber, a nylon fiber, a rayon fiber or a polyethylene naphthalate fiber. In order to obtain a tire which is excellent in a stiffness feeling, therefore, a cord constituted by a yarn formed of the aramid fiber is used.
The yarn formed by the aramid fiber usually has a fineness of 1100 dtex or 1670 dtex. In a tire using a cord constituted by a yarn having a fineness of 1100 dtex, the cord causes a fatigue break in some cases. The tire has a poor durability. In a tire using a cord constituted by a yarn having a fineness of 1670 dtex, the fatigue break of the cord can be prevented. The cord can contribute to a durability of the tire. However, the cord has a great weight. For this reason, the cord influences the weight of the tire. Furthermore, the carcass including the cord has an excessive stiffness. Therefore, the carcass influences a following property to a road surface and a stiffness feeling in the tire.
It is an object of the present invention to provide a motorcycle tire for an off-road which is excellent in a following property to a road surface, a stiffness feeling and a durability.